The composition distribution of an ethylene alpha-olefin copolymer refers to the distribution of comonomer (short chain branches) among the molecules that comprise the polyethylene polymer. When the amount of short chain branches varies among the polyethylene molecules, the resin is said to have a “broad” composition distribution. When the amount of comonomer per 1000 carbons is similar among the polyethylene molecules of different chain lengths, the composition distribution is said to be “narrow”.
The composition distribution is known to influence the properties of copolymers, for example, extractables content, environmental stress crack resistance, heat sealing, and tear strength. The composition distribution of a polyolefin may be readily measured by methods known in the art, for example, Temperature Raising Elution Fractionation (TREF) or Crystallization Analysis Fractionation (CRYSTAF).
Ethylene alpha-olefin copolymers are typically produced in a low pressure reactor, utilizing, for example, solution, slurry, or gas phase polymerization processes. Polymerization takes place in the presence of catalyst systems such as those employing, for example, a Ziegler-Natta catalyst, a chromium based catalyst, a metallocene catalyst, or combinations thereof.
It is generally known in the art that a polyolefin's composition distribution is largely dictated by the type of catalyst used and typically invariable for a given catalyst system. Ziegler-Natta catalysts and chromium based catalysts produce resins with broad composition distributions (BCD), whereas metallocene catalysts normally produce resins with narrow composition distributions (NCD).
Resins having a Broad Orthogonal Composition Distribution (BOCD) in which the comonomer is incorporated predominantly in the high molecular weight chains can lead to improved physical properties, for example toughness properties and Environmental Stress Crack Resistance (ESCR).
Because of the improved physical properties of resins with orthogonal composition distributions needed for commercially desirable products, there exists a need for medium and high density polyethylenes having an orthogonal composition distribution.